Wow I've actually just started working on something quite similar to the very first app listed here. It's somewhat discouraging to say the least. The travel social network market is rather large - there are a few existing sites that do an okay job at it and get fairly decent traffic numbers but none of them are clearly great. At least none were until I clicked on that link.

Edit: It's actually kind of odd for how nice the site is, it's virtually unfindable on Google. Even if it takes time to gain PageRank you could at least start advertising it with AdWords...